\documentclass[10pt]{amsart}

\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{fullpage}
\usepackage{graphicx}
\usepackage{tikz}
\usepackage[all]{xy}

\renewcommand{\familydefault}{\sfdefault}
\setlength\parindent{0cm}

\newtheorem*{defn}{Definition}
\DeclareMathOperator{\ord}{ord}
\DeclareMathOperator{\Aut}{Aut}
\DeclareMathOperator{\Prob}{Prob}

\title{Theorem }
\date{}

\begin{document}
\maketitle

\subsection*{Statement}

\subsection*{Outline of the proof}

\subsection*{Detailed proof}

\end{document}